1. Field of the Invention
One embodiment of the present invention relates to an imaging device.
Note that one embodiment of the present invention is not limited to the above technical field. The technical field of one embodiment of the invention disclosed in this specification and the like relates to an object, a method, or a manufacturing method. In addition, one embodiment of the present invention relates to a process, a machine, manufacture, or a composition of matter. Specifically, examples of the technical field of one embodiment of the present invention disclosed in this specification include a semiconductor device, a display device, a liquid crystal display device, a light-emitting device, a lighting device, a power storage device, a storage device, an imaging device, a method for driving any of them, and a method for manufacturing any of them.
In this specification and the like, a semiconductor device means a general device that can function by utilizing semiconductor characteristics. A transistor and a semiconductor circuit are embodiments of semiconductor devices. In some cases, a storage device, a display device, an imaging device, or an electronic device includes a semiconductor device.
2. Description of the Related Art
In medical practice, a medical diagnostic imaging device using a photographic technique has been in wide use. With the medical diagnostic imaging device, a specific portion of a patient is irradiated with X-rays, an X-ray film is exposed to X-rays that have passed through the specific portion, and the X-ray film is developed to visualize a state inside the specific portion.
Since the method using X-ray films needs a storage space for the X-ray films and maintenance thereof is troublesome, digitization of images is in progress. As a method for digitizing images, a method using an imaging plate is known. An imaging plate is irradiated with X-rays in a manner similar to that of the X-ray films and light emitted from the imaging plate is sensed with a scanner; thus, digitized images can be obtained.
The imaging plate is a plate to which a material having a property of emitting light by X-ray irradiation (this property is referred to as photostimulability, and the material having this property is referred to as a photostimulable phosphor) is applied and has higher detection sensitivity to X-ray absorption difference than an X-ray film does. In addition, data of X-ray irradiation can be erased, so that the imaging plate can be re-used. However, the data obtained by the imaging plate is analog, so that digitization treatment to digitize the data is necessary.
For this reason, attention has been recently focused on flat panel detectors capable of obtaining digital data directly (e.g., Patent Document 1). Flat panel detectors have two types of systems, direct and indirect conversion systems. In the direct conversion system, X-rays are directly converted to electrical charges with the use of an X-ray detecting element. In the indirect conversion system, X-rays are converted to visible light with a scintillator and the light is converted to electrical charges by a photodiode. In either of the systems, a flat panel detector includes a plurality of pixel circuits arranged in a matrix.